You've parented the the ribs to the armature, so accidentally the arm acts on the ribs and you need to correct that:

• Select the arm to know its name:

• Select the ribs, go in the Properties panel > Object Data > Vertex Groups, find the vertex groups that has the same name, click on Remove:

The lower arm influence the ribs as well so do the same. You can also do it in Weight Paint mode.

You've parented the the ribs to the armature, so accidentally the arm acts on the ribs and you need to correct that:

• Select the arm to know its name:

• Select the ribs, go in the Properties panel > Object Data > Vertex Groups, find the vertex groups that has the same name, click on Remove:

The lower arm influences the ribs as well so do the same. You can also do it in Weight Paint mode.